Uh-oh! Tube help

Is there any way to visually tell a 12ax7 from a 12 au7? Well, I had my Ampeg preamp serviced for buzz a while back, and they replaced 3 of the tubes, including the Au7. I noticed that all the replaced and new tubes have a kind of double stack in the middle... while the au7 has a single stack. I think they replaced the au7 with an ax7. The replacement tubes are totally unlabeled.

Now, I've been experimenting, and there doesn't seem to be a difference between an ax7 and an au7 in that slot, but it's a little disturbing.

I'm sending back to the service center anyway because of a totaly different issue (distortion/weak signal that goes away after a few minutes of being on and only comes back after it's off for a while)

If you know what brand they are, there MIGHT be a way to tell, but quite frankly if you're not hearing a difference in your sound, don't worry about it. A 12AX7 and a 12AU7 are completely pin compatible so there's so chance of damage from the swap. A 12AX7 has higher gain, that's all.

I'll bet your tubes are Sovtek. Their writing is VERY light and hard to see and they are common for replacements. SLM uses them a bunch. Take another CLOSE careful look at the tubes and see if they don't have some sort of markings.